Rubber composition and solid golf ball

ABSTRACT

Disclosed is a rubber composition containing (a) 50 parts to 100 parts by weight of polybutadiene rubber having a 1,4-cis bond content of 80 mol % to 100 mol % and a 1,2-vinyl bond content of 0 mol % to 2 mol %, which is modified with an alkoxysilyl group-containing compound, (b) 0 part to 50 parts by weight of diene rubber other than the above-mentioned component (a) (wherein the total amount of component (a) and component (b) is 100 parts by weight), (c) 10 parts to 50 parts by weight of a crosslinkable monomer, (d) 5 parts to 80 parts by weight of an inorganic filler and (e) 0.1 part to 10 parts by weight of an organic peroxide, and a solid golf ball improved in flight, durability and the feeling of ball hitting can be produced from the rubber composition.

FIELD OF THE INVENTION

[0001] The present invention relates to a rubber composition and a solidgolf ball. More particularly, the present invention relates to a rubbercomposition giving a solid golf ball good in the feeling of ballhitting, long in flight (carry) and excellent in durability, and a solidgolf ball obtained therefrom.

BACKGROUND OF THE INVENTION

[0002] Solid golf balls include one-piece solid golf balls integrallymolded from crosslinked products of rubber compositions, andmultiple-pieces solid golf balls such as two-pieces solid golf balls,three-pieces solid golf balls and four-pieces solid golf balls in whichsolid cores having one- to three-layers structure composed ofcrosslinked products of hard rubber compositions are coated with covers.

[0003] Of these solid golf balls, the multiple-pieces solid golf ballsare particularly excellent in flight, so that in recent years, they havemainly been used as golf balls for playing rounds of golf. However, themultiple-pieces solid golf balls have the disadvantage that the feelingof ball hitting is hard, compared with conventional thread-woundstructure golf balls. Then, it has been attempted that the cores aresoftened, and moreover, portions nearer to the centers of the cores aremore softened to increase the deformation of the cores on hitting,thereby improving the feeling of ball hitting of the multiple-piecessolid golf balls. However, the softening of the cores deterioratesdurability and resilience (carry). It has been therefore desired thatmultiple-pieces solid golf balls good in the feeling of ball hitting,long in flight and excellent in durability appear.

[0004] On the other hand, the one-piece solid golf balls have mainlybeen used as golf balls for training fields, and are liable to becracked or chipped by repeated hitting. Accordingly, excellentdurability is required so as to prevent the balls from being cracked orchipped as far as possible. Further, golf training players require thatthe feeling of ball hitting is also good.

[0005] Rubber compositions containing polybutadiene having a 1,4-cisbond content of 80 mol % or more, which is synthesized using a nickelcatalyst or a cobalt catalyst, have hitherto suitably been used forcores of the multiple-pieces solid golf balls and core portions (solidcenters) of the one-piece solid golf balls, because of their highresilience and durability. It is further known that polybutadienesynthesized using a rare earth element catalyst can be used for asimilar purpose.

[0006] For example, Examined Japanese Patent Publication (Hei) 3-59931,Examined Japanese Patent Publication (Hei) 6-80123, Japanese Patent No.2678240, Japanese Patent Laid-open Publication (Hei) 6-79018 andJapanese Patent Laid-open Publication (Hei) 11-319148 disclose thatrubber compositions containing polybutadiene synthesized using rareearth element catalysts are suitable for the golf ball purpose. However,the resilience and durability of the resulting golf balls areinsufficient. Further, the production workability thereof is alsoinsufficient.

[0007] Japanese Patent Laid-open Publication (Hei) 7-268132 discloses agolf ball composed of a rubber composition mainly containingpolybutadiene synthesized using a rare earth element catalyst modifiedwith a tin compound. However, the ratio (Mw/Mn) of weight averagemolecular weight (Mw) to number average molecular weight (Mn) is high,so that the resilience and durability are insufficient.

[0008] Japanese Patent Laid-open Publication (Hei) 11-164912 disclosesthe use of modified polybutadiene rubber having a 1,4-cis bond contentof 80 mol % or more, a 1,2-vinyl bond content of 2 mol % or less and alow ratio (Mw/Mn) of weight average molecular weight (Mw) to numberaverage molecular weight (Mn) of 3.5 or less. However, this isinsufficient in production workability, and there is still room forimprovement in resilience and durability.

SUMMARY OF THE INVENITON

[0009] It is therefore an object of the present invention to provide arubber composition which can give a solid golf ball improved in flight,durability and the feeling of ball hitting.

[0010] Another object of the present invention is to provide a solidgolf ball improved in flight, durability and the feeling of ballhitting.

[0011] According to the present invention, there are provided thefollowing rubber composition and solid golf ball, thereby achieving theabove-mentioned objects of the present invention.

[0012] (1) A rubber composition containing (a) 50 parts to 100 parts byweight of polybutadiene rubber having a 1,4-cis bond content of 80 mol %to 100 mol % and a 1,2-vinyl bond content of 0 mol % to 2 mol %, whichis modified with an alkoxysilyl group-containing compound, (b) 0 part to50 parts by weight of diene rubber other than the above-mentionedcomponent (a) (wherein the total amount of component (a) and component(b) is 100 parts by weight), (c) 10 parts to 50 parts by weight of acrosslinkable monomer, (d) 5 parts to 80 parts by weight of an inorganicfiller and (e) 0.1 part to 10 parts by weight of an organic peroxide;

[0013] (2) The rubber composition described in the above (1), whereinmodified polybutadiene rubber (a) has a ratio (Mw/Mn) of weight averagemolecular weight (Mw) to number average molecular weight (Mn) of 1.0 to3.5;

[0014] (3) The rubber composition described in the above (1) or (2),wherein modified polybutadiene rubber (a) has a Mooney viscosity (ML₁₊₄(100° C.)) of 30 to 100;

[0015] (4) The rubber composition described in any one of the above (1)to (3), wherein component (a) is modified polybutadiene rubber obtainedby polymerizing butadiene using a rare earth element catalyst, andsubsequently allowing the alkoxysilyl group-containing compound to reactwith the resulting polybutadiene;

[0016] (5) The rubber composition described in the above (4), whereinthe rare earth element catalyst is a neodymium catalyst;

[0017] (6) The rubber composition described in any one of the above (1)to (5), wherein the alkoxysilyl group-containing compound is analkoxysilane compound having at least one epoxy group or isocyanategroup in a molecule thereof;

[0018] (7) The rubber composition described in any one of the above (1)to (6), wherein the alkoxysilyl group-containing compound is3-glycidyloxypropyltrimethoxysilane; and

[0019] (8) A solid golf ball in which a part or the whole of a rubbersubstance constituting the solid golf ball is obtained by crosslinkingand molding the rubber composition described in any one of the above (1)to (7).

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a schematic cross sectional view showing an example ofone-piece solid golf balls;

[0021]FIG. 2 is a schematic cross sectional view showing an example oftwo-pieces solid golf balls; and

[0022]FIG. 3 is a schematic cross sectional view showing an example ofthree-pieces solid golf balls.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] First, component (a) of the rubber composition of the presentinvention will be described.

[0024] As described above, the modified polybutadiene rubber modifiedwith the alkoxysilyl group-containing compound, component (a), has a1,4-cis bond content (cis content) of 80 mol % to 100 mol %, preferably90 mol % to 100 mol %, and a 1,2-vinyl bond content (vinyl content) of 0mol % to 2 mol %, preferably 0 mol % to 1.5 mol %. As to component (a),the ratio (Mw/Mn) of weight average molecular weight (Mw) to numberaverage molecular weight (Mn) is preferably from 1.0 to 3.5, and morepreferably from 1.0 to 3.3; the Mooney viscosity (ML₁₊₄ (100° C.)) ispreferably from 30 to 100, and more preferably from 50 to 90; and theviscosity of a 5-wt % toluene solution (SV: solvent viscosity) ispreferably from 150 to 1,500 cPs, and more preferably from 200 to 1,500cPs.

[0025] A 1,4-cis bond content (cis content) of modified polybutadienerubber (a) of less than 80 mol % results in inferior resilience.Further, a 1,2-vinyl bond content (vinyl content) exceeding 2 mol % alsoresults in inferior resilience.

[0026] Still further, an Mw/Mn of component (a) exceeding 3.5 results ininferior resilience and durability. A Mooney viscosity (ML₁₊₄ (100° C.))of component (a) of less than 30 results in inferior resilience anddurability, whereas exceeding 100 results in inferior productionworkability. Furthermore, when the viscosity of a 5-wt % solution ofcomponent (a) in toluene at 25° C. (SV) is less than 150 cPs, theresilience is deteriorated.

[0027] The modification of polybutadiene with the alkoxysilylgroup-containing compound is conducted, for example, by adding thealkoxysilyl group-containing compound to a polybutadiene rubber solutionafter polymerization to allow the compound to react with polybutadiene.

[0028] The above-mentioned modified polybutadiene rubber is preferablyproduced by polymerizing butadiene in the presence of a rare earthelement catalyst showing quasi-living polymerizability, and subsequentlyallowing the alkoxysilyl group-containing compound to react with theresulting polybutadiene.

[0029] The rubber composition of the present invention containspolybutadiene having mostly 1,4-cis bonds, as a main rubber component,moreover, narrow in molecular weight distribution and high in solutionviscosity, and the polybutadiene is modified with the alkoxysilylgroup-containing compound, thereby obtaining the rubber compositionexcellent in production workability. Further, the solid golf ball havinga rubber substance constituted by a crosslinked molded article of therubber composition of the present invention shows long flight, and isexcellent in the feeling of ball hitting and durability.

[0030] As the rare earth element catalyst used in the polymerization ofbutadiene, a known catalyst can be used.

[0031] For example, there can be used lanthanum series rare earthelement compounds, organic aluminum compounds, alumoxane,halogen-containing compounds and a combination with Lewis bases inoption.

[0032] The lanthanum series rare earth element compounds include halidesof metals of atomic numbers 57 to 71, carboxylates, alcoholates,thioalcoholates and amides.

[0033] Further, as the organic aluminum compounds, there are usedcompounds represented by AlR¹R²R³ (wherein R¹, R² and R³, which may bethe same or different, each represent hydrogen or hydrocarbon residueseach having 1 to 8 carbon atoms). The alumoxanes are compounds having astructure represented by the following formula (I) or (II). Aggregatesof the alumoxanes described in Fine Chemical, 23 (9), 5 (1994), J. Am.Chem. Soc., 115, 4971 (1993), and J. Am. Chem. Soc., 117, 6465 (1995)may be used.

[0034] wherein R⁴ is a hydrocarbon group having 1 to 20 carbon atoms,and n is an integer of 2 or more.

[0035] As the halogen-containing compounds, there are used aluminumhalides represented by AlX_(n)R⁵ _(3−n) (wherein X is a halogen atom, R⁵is a hydrocarbon residue having 1 to 20 carbon atoms such as an alkylgroup, an aryl group or an aralkyl group, and n is 1, 1.5, 2 or 3);strontium halides such as Me₃SrCl, Me₂SrCl₂, MeSrHCl₂ and MeSrCl₃; andmetal halides such as silicon tetrachloride, tin tetrachloride andtitanium tetrachloride. The Lewis bases are used for complexing thelanthanum series rare earth element compounds. For example,acetylacetone and ketone alcohols are suitably used. Above all,neodymium catalysts in which neodymium compounds are used as thelanthanum series rare earth element compounds are preferably used,because polybutadiene rubber high in the 1,4-cis bond content and low inthe 1,2-vinyl bond content is obtained with excellent polymerizationactivity.

[0036] Specific examples of these rare earth element catalysts aredescribed in Examined Japanese Patent Publication (Sho) 62-1404,Examined Japanese Patent Publication (Sho) 63-64444, and Japanese PatentLaid-open Publication (Hei) 11-35633, Japanese Patent Laid-openPublication (Hei) 10-306113 and Japanese Patent Laid-open Publication2000-34320 filed by the present applicant, and can be used.

[0037] When butadiene is polymerized in the presence of the rare earthelement catalyst using the lanthanum series rare earth element compound(La type compound), the molar ratio of butadiene to the La type compoundis preferably from 1,000 to 2,000,000, and particularly preferably from5,000 to 1,000,000, and the molar ratio of AlR¹R²R³ to the La typecompound is preferably from 1 to 1,000, and particularly preferably from3 to 500, for obtaining a cis content and a Mw/Mn ratio within theabove-mentioned ranges. Further, the molar ratio of the halogen compoundto the La type compound is preferably from 0.1 to 30, and particularlypreferably from 0.2 to 15. The molar ratio of the Lewis base to the Latype compound is preferably from 0 to 30, and particularly preferablyfrom 1 to 10. In polymerization, a solvent may be used, or bulkpolymerization or gas phase polymerization using no solvent may beemployed. The polymerization temperature is usually from −30° C. to 150°C., and preferably from 10° C. to 100° C.

[0038] The modified polybutadiene rubber is obtained by allowing thealkoxysilyl group-containing compound (terminal modifier) to react withan active terminal of the polymer, following after the abovepolymerization.

[0039] As the alkoxysilyl group-containing compound, there is suitablyused an alkoxysilane compound having at least one epoxy group orisocyanate group in its molecule. Specific examples thereof includeepoxy group-containing alkoxysilane compounds such as3-glycidyloxypropyl-trimethoxysilane,3-glycidyloxypropyltriethoxysilane,(3-glycidyloxypropyl)methyldimethoxysilane,(3-glycidyl-oxypropyl)methyldiethoxysilane, β-(3,4-epoxycyclohexyl)trimethoxysilane, β-(3,4-epoxycyclohexyl)triethoxysilane,β-(3,4-epoxycyclohexyl)methyldimethoxysilane,β-(3,4-epoxycyclohexyl)ethyldimethoxysilane, a condensate of3-glycidyloxypropyltrimethoxysilane and a condensate of(3-glycidyloxypropyl)methyldimethoxysilane; and isocyanategroup-containing alkoxysilane compounds such as3-isocyanatepropyltrimethoxysilane, 3-isocyanatepropyl-triethoxysilane,(3-isocyanatepropyl)methyldimethoxy-silane,(3-isocyanatepropyl)methyldiethoxysilane, a condensate of3-isocyanatepropyltrimethoxysilane and a condensate of(3-isocyanatepropyl)methyldimethoxysilane.

[0040] Further, when the above-mentioned alkoxysilyl group-containingcompound is allowed to react with the active terminal of the polymer, aLewis acid can also be added for accelerating the reaction. The Lewisacid accelerates the coupling reaction as a catalyst thereby improvingthe cold flow of the modified polymer, resulting in the improved storagestability thereof.

[0041] Specific examples of the Lewis acids include dialkyltindialkylmaleates, dialkyltin dicarboxylates and aluminum trialkoxides.

[0042] As reaction methods of modification with the above-mentionedterminal modifiers, there can be used known methods per se. for example,a method described in Japanese Patent Laid-open Publication (Hei)11-35633 filed by the present applicant and a method described inJapanese Patent Laid-open Publication (Hei) 7-268132 can be employed.

[0043] Then, the diene rubber other than the above-mentioned component(a), component (b), will be illustrated below.

[0044] Component (b) is not indispensable to the rubber composition ofthe present invention, and a component optionally added within the rangenot inhibiting the achievement of the objects of the present invention.Specific examples of components (b) include unmodified or modifiedpolybutadiene rubber having a cis content of less than 80 mol % or anMw/Mn ratio exceeding 3.5, styrene-butadiene rubber (SBR), naturalrubber, synthetic polyisoprene rubber and ethylene-propylene-dienerubber (EPDM).

[0045] These can be used either alone or as a combination of two or moreof them.

[0046] Then, the crosslinkable monomer, component (c), will be describedbelow.

[0047] Crosslinkable monomer (c) is polymerized by radicals produced bydecomposition of organic peroxides described below and functioning asradical initiators, and acts so as to accelerate the crosslinking ofcomponent (a) and component (b).

[0048] The crosslinkable monomers added to the rubber compositions ofthe present invention are preferably monovalent or divalent metal saltsof α,β-ethylenic unsaturated carboxylic acids, and specific examplesthereof include the following:

[0049] (i) Acrylic acid, methacrylic acid, itaconic acid, maleic acid,fumalic acid, crotonic acid, sorbic acid, tiglic acid, cinnamic acid andaconitic acid (these can be used either alone or as a combination of twoor more of them); and

[0050] (ii) Zn, Ca, Mg, Ba and Na salts of the unsaturated acidsdescribed in the above (i) (these can be used either alone or as acombination of two or more of them).

[0051] The unsaturated acids of the above (i) and the metal salts of theabove (ii) can be used in combination. The above-mentioned metal saltsof the α,β-ethylenic unsaturated carboxylic acids may be either mixed assuch with base rubber by conventional methods, or formed by adding theα,β-ethylenic unsaturated carboxylic acids such as acrylic acid andmethacrylic acid to the rubber compositions with which metal oxides suchas zinc oxide have previously been mixed by kneading them, and kneadingthe α,β-ethylenic unsaturated carboxylic acids and the rubbercompositions together to allow the α,β-ethylenic unsaturated carboxylicacids to react with the metal oxides in the rubber compositions. Thecrosslinkable monomers (c) can be used either alone or as a combinationof two or more of them.

[0052] The inorganic filler, component (d) will be illustrated below.

[0053] Inorganic filler (d) can reinforce the crosslinked rubber toimprove the strength, and adjust the weight of the solid golf ball bythe amount thereof added.

[0054] Specifically, the inorganic fillers include zinc oxide, bariumsulfate, silica, alumina, aluminum sulfate, calcium carbonate, aluminumsilicate and magnesium silicate. Zinc oxide, barium sulfate and silicaare preferably used among others. These inorganic fillers can be usedeither alone or as a combination of two or more of them.

[0055] Then, the organic peroxide, component (e), will be illustratedbelow.

[0056] Organic peroxide (e) added to the rubber composition of thepresent invention acts as an initiator for crosslinking reaction, graftreaction and polymerization reaction of rubber components comprisingcomponent (a) and component (b), and crosslinkable monomer (c).

[0057] Suitable specific examples of the organic peroxides includedicumyl peroxide, 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane,2,5-dimethyl-2,5-di-(t-butylper-oxy)hexane and1,3-bis(t-butylperoxyisopropyl)benzene.

[0058] The amount ratio of component (a), component (b), crosslinkablemonomer (c), inorganic filler (d) and organic peroxide (e) is asfollows.

[0059] The amount of modified polybutadiene rubber (a) is from 50 partsto 100 parts by weight, and preferably from 60 parts to 100 parts byweight, and the amount of diene rubber (b) other than theabove-mentioned component (a) is from 0 part to 50 parts by weight, andpreferably from 0 part to 40 parts by weight (wherein the total amountof component (a) and component (b) is 100 parts by weight). Incomponents (a) and (b), when the amount of component (a) added is lessthan 50 parts by weight, the resilience of the solid golf ball isinsufficient, which unfavorably causes no increase in initial velocityof the ball and no prolongation in flight (carry).

[0060] The amount of crosslinkable monomer (c) added is from 10 parts to50 parts by weight, and preferably from 10 parts to 40 parts by weight,based on 100 parts by weight of the total amount of components (a) and(b). Less than 10 parts by weight results in insufficient resilience ofthe solid golf ball, which causes a decrease in initial velocity of thegolf ball and a decrease in flight, whereas exceeding 50 parts by weightresults in so hard a golf ball that the feeling of ball hitting isdeteriorated.

[0061] The amount of inorganic filler (d) added is from 5 parts to 80parts by weight, and preferably from 5 parts to 70 parts by weight,based on 100 parts by weight of the total amount of components (a) and(b). Less than 5 parts by weight results in too light a solid golf ballobtained, whereas exceeding 80 parts by weight results in too heavy asolid golf ball obtained.

[0062] The amount of organic peroxide (e) added is from 0.1 part to 10parts by weight, and preferably from 0.2 part to 5 parts by weight,based on 100 parts by weight of the total amount of components (a) and(b). Less than 0.1 part by weight results in too soft a golf ball, whichcauses deterioration in the resilience to decrease flight (carry),whereas exceeding 10 parts by weight results in too hard a golf ball,which causes deterioration in the feeling of ball hitting.

[0063] By allowing the above-mentioned respective components to becontained in amounts within the above-mentioned ranges, the solid golfball excellent in flight, durability and the feeling of ball hitting isobtained from the rubber composition of the present invention.

[0064] The rubber composition of the present invention may contain acrosslinking assistant such as zinc oxide, a lubricant such as stearicacid, and an antioxidant, as desired, in addition to the above-mentionedcomponents (a) and (b), crosslinkable monomer (c), inorganic filler (d)and organic peroxide (e).

[0065] Typical examples of the solid golf balls produced from the rubbercompositions of the present invention by crosslinking and molding willbe illustrated with reference to the drawings.

[0066]FIG. 1 is a schematic cross sectional view showing a one-piecesolid golf ball. The reference numeral 1 designates a main body portion,and the reference numeral la designates dimples. The main body portion 1is constituted by a rubber substance (that is to say, a rubber substancecomposed of a crosslinked molded article of the rubber composition ofthe present invention).

[0067]FIG. 2 is a schematic cross sectional view showing a two-piecessolid golf ball. The reference numeral 11 designates a core, and thereference numeral 12 designates a cover. The core 11 is coated with thecover 12, and the reference numeral 12 a designates dimples. The core 11is constituted by a rubber substance.

[0068]FIG. 3 is a schematic cross sectional view showing a three-piecessolid golf ball. The reference numeral 21 designates an inner core, thereference numeral 22 designates an outer core, the reference numeral 23designates a cover, and the reference numeral 23 a designates dimples.In this three-pieces solid golf ball, the inner core 21 and the outercore 22 constitute a solid core. The inner core 21 or the outer core 22,or both the inner core 21 and the outer core 22 are constituted by arubber substance. It is preferred in terms of flight and rotationkeeping properties that the density of the outer core 22 of thethree-pieces solid golf ball is higher than that of the inner core 21.For example, the above can be achieved by adding a filler having ahigher specific gravity such as W₂O₅ to the outer core 22 and a fillerhaving a lower specific gravity such as ZnO₂ to the inner core 21.

[0069] Then, methods for producing the solid golf balls using the rubbercompositions of the present invention will be described. First, therubber composition of the present invention is placed in a requiredmold, and crosslinked and molded with a press to form each of the mainbody portion of the one-piece solid golf ball, the core of thetwo-pieces solid golf ball and the inner core of three-pieces solid golfball. The crosslinking is preferably conducted at a temperature of 130°C. to 180° C. for 10 minutes to 50 minutes. The temperature incrosslinking and molding may be changed in two or more steps.

[0070] In the three-pieces solid golf ball, a sheet of the rubbercomposition for the outer layer, which is formed to a desired thickness,is adhered to the outside of the inner core obtained as described above,and crosslinked and molded with a press to form a solid core having atwo-layers structure. In the three-pieces solid golf ball, either therubber composition used for the inner core or the rubber compositionused in the outer core should be the rubber composition of the presentinvention. However, it is preferred that both are the rubber compositionof the present invention.

[0071] The covers of the two-pieces solid golf ball and the three-piecessolid golf ball are formed by coating the above-mentioned cores withcover compositions in which additives, for example, inorganic whitepigments such as titanium oxide, and light stabilizers, areappropriately mixed with resin components mainly composed of ionomerresins or the like. In coating, the injection molding method is usuallyemployed. However, the coating method is not limited thereto.

[0072] Further, desired dimples are formed as needed, in molding themain body portion for the one-piece solid golf ball, and in molding thecover for the two-pieces solid golf ball or the three-pieces solid golfball.

[0073] A four-pieces solid golf ball can also be produced from thecomposition of the present invention in the same manner as with thethree-pieces solid golf ball.

[0074] The solid golf balls obtained from the rubber compositions of thepresent invention are excellent in production workability, good in thefeeling of ball hitting, long in flight and excellent in durability.

[0075] This invention will be illustrated with reference to examples inmore details below, but the following disclosure shows preferredembodiments of the invention and not intended to limit the scope of theinvention.

SYNTHESIS EXAMPLE 1 Modified Polybutadiene Rubber (A): Synthesis of HPB(A)

[0076] An autoclave having an inner volume of 5 liters, the inside ofwhich was replaced with nitrogen, was charged with 2.5 kg of cyclohexaneand 300 g of 1,3-butadiene under a nitrogen atmosphere. A catalyst wasprepared by previously mixing a solution of neodymium octanoate (0.18mmol) and acetylacetone (0.37 mmol) in cyclohexane, a solution ofmethylalumoxane (18.5 mmol) in toluene, a solution of diisobutylaluminumhydride (3.9 mmol) in cyclohexane and a solution of diethylaluminumchloride (0.370 mmol) in cyclohexane, and reacting the resulting mixturewith 5-fold moles of 1,3-butadiene (0.90 mmol) in relation to neodymiumoctanoate and aging the resulting product at 25° C. for 30 minutes. Theabove prepared catalyst was added to the mixture in the autoclave,followed by polymerization at 50° C. for 30 minutes. The degree ofconversion of 1,3-butadiene was approximately 100%. Then, thetemperature of the polymerization solution was kept at 50° C., and3-glycidyl-oxypropyltrimethoxysilane (5.40 mmol) was added thereto.Thereafter, the solution was allowed to stand for 30 minutes, and asolution of 2,4-di-t-butyl-p-cresol (1.5 g) in methanol was added. Afterthe termination of polymerization, the solvents were removed by steamstripping, and the resulting product was dried on a roll heated at 110°C. to obtain a polymer. The 1,4-cis bond content of the obtained polymerwas 97.8 mol %, the 1,2-vinyl bond content was 1.0 mol %, the Mw/Mnratio was 2.4, the Mooney viscosity (ML₁₊₄ (100° C.)) was 41, and thetoluene solution viscosity was 210.

SYNTHESIS EXAMPLE 2 Modified Polybutadiene Rubber (B): Synthesis of HPB(B)

[0077] A polymer was prepared in the same manner as with the synthesisexample of HPB (A) with the exception that the amount of methylalumoxaneadded was changed to 9.2 mmol. The property values of the polymer areshown in Table 1.

SYNTHESIS EXAMPLE 3 Modified Polybutadiene Rubber (C): Synthesis of HPB(C)

[0078] A polymer was prepared in the same manner as with the synthesisexample of HPB (A) with the exception that the amount of methylalumoxaneadded was changed to 9.2 mmol, and the amount of diisobutylaluminumhydride added was changed to 4.5 mmol. The property values of thepolymer are shown in Table 1.

SYNTHESIS EXAMPLE 4 Modified Polybutadiene Rubber (D): Synthesis of HPB(D)

[0079] A polymer was prepared in the same manner as with the synthesisexample of HPB (A) with the exception that the amount of methylalumoxaneadded was changed to 9.2 mmol, and the amount of diisobutylaluminumhydride added was changed to 3.2 mmol. The property values of thepolymer are shown in Table 1.

SYNTHESIS EXAMPLE 5 Modified Polybutadiene Rubber (E): Synthesis of HPB(E)

[0080] A polymer was prepared in the same manner as with the synthesisexample of HPB (A) with the exception that the amount of methylalumoxaneadded was changed to 9.2 mmol, and the amount of diisobutylaluminumhydride added was changed to 5.0 mmol. The property values of thepolymer are shown in Table 1.

SYNTHESIS EXAMPLE 6 Modified Polybutadiene Rubber (F): Synthesis of HPB(F)

[0081] A polymer was prepared in the same manner as with the synthesisexample of HPB (A) with the exception that the amount of methylalumoxaneadded was changed to 9.2 mmol, and the amount of diisobutylaluminumhydride added was changed to 1.6 mmol. The property values of thepolymer are shown in Table 1.

SYNTHESIS EXAMPLE 7 Modified Polybutadiene Rubber (G): Synthesis of HPB(G)

[0082] A polymer was prepared in the same manner as with the synthesisexample of HPB (A) with the exception that the amount of methylalumoxaneadded was changed to 5.1 mmol. The property values of the polymer areshown in Table 1.

SYNTHESIS EXAMPLE 8 Modified Polybutadiene Rubber (H): Synthesis of HPB(H)

[0083] A polymer was prepared in the same manner as with the synthesisexample of HPB (A) with the exception that the amount of methylalumoxaneadded was changed to 9.2 mmol, and the modifier was changed to polymerictype diphenylmethane diisocyanate (5.4 mmol). The property values of thepolymer are shown in Table 1.

[0084] HPB (H) is modified polybutadiene rubber for comparison using themodifier (polymeric type diphenylmethane diisocyanate) described inJapanese Patent Laid-open Publication (Hei) 11-164912. BR01, BR03, BR11and BR18 in Table 1 are unmodified polybutadiene rubber using nomodifier.

[0085] As to the solution viscosity (SV) in Table 1, the viscosity of a5-wt % solution of a previously prepared polymer in toluene was measuredwith a Cannon-Fenske viscometer in a thermostat at 25° C. TABLE 1Synthesis Example 1 2 3 4 5 6 7 8 Polybutadiene HPB HPB HPB HPB HPB HPBHPB HPB BR01 BR03 BR11 BR18 (A) (B) (C) (D) (E) (F) (G) (H)Polymerization Conditions Polymerization Nd Nd Nd Nd Nd Nd Nd Nd Ni NiNi Ni Cat. Si Si Si Si Si Si Si MDI Unmod Unmod Unmod Unmod Modifierified ified ified ified Properties Mooney Viscosity 41 40 35 56 28 10543 44 45 34 43 59 1,4-Cis Content 97.8 98.1 97.6 98.3 98.1 98.5 97.697.5 95 94.5 96 96 1,2-Vinyl 1 1.1 1.2 1 1 1.1 1.1 1.1 2.5 2.5 2.5 2.0Content 210 295 165 390 115 1120 360 330 150 75 270 590 SolutionViscosity (SV) 2.4 3.2 2.9 3.1 3.1 2.9 4.1 3.1 4.0 2.4 4.3 4.4 Mw/Mn

EXAMPLES 1 TO 6 AND COMPARATIVE EXAMPLES 1 TO 4

[0086] According to a formulation described in Table 2 shown below,polybutadiene shown in Table 1, zinc diacrylate, zinc oxide, dicumylperoxide and the antioxidant were kneaded with rolls, and the resultingrubber composition was crosslinked and molded under pressure at 150° C.for 30 minutes to obtain a core having a diameter of 38.5 mm. Forcomparing production workability, the wrapping test was carried outusing 6-inch rolls to evaluate roll processability.

[0087] Conditions of the wrapping test; temperature: 70° C., nip width:1.4 mm, revolutions: 20 rpm/24 rpm Here, the roll processability wasevaluated as follows. The larger numerical value indicates the betterroll processability (the same applies in Tables 3 and 4).

[0088] 5: The rubber composition finely wraps around the roll, and asurface thereof is smooth.

[0089] 4: The rubber composition wraps around the roll, and a surfacethereof does not feel rough.

[0090] 3: The rubber composition wraps around the roll, but a surfacethereof feels rough.

[0091] 2: The rubber composition wraps around the roll, but one or moreholes are formed on a surface thereof to cause a poor appearance.

[0092] 1: The rubber composition does not wrap around the roll.

[0093] Results shown in Table 2 reveal that Examples 1 to 6 are superiorto Comparative Examples 1 to 4 in roll processability.

[0094] Then, the resulting core was coated by the injection moldingmethod with a cover composition composed of a mixture of 100 parts byweight of an ionomer resin (trade name: Surlyn, manufactured by du Pont)and 2 parts by weight of titanium dioxide to form a cover, therebypreparing a two-pieces solid golf ball having an outer diameter of 42.7mm. In Comparative Examples 2 and 3, the modifier (polymeric typediphenylmethane diisocyanate) described in Japanese Patent Laid-openPublication (Hei) 11-164912 was used. The ball obtained in ComparativeExample 4 is a conventional standard two-pieces solid golf ball.

[0095] As to the two-pieces solid golf balls thus obtained, theirweight, compression (PGA indication), initial velocity, flight andhammering durability were measured. Results thereof are shown in Table2. Further, ten top professional golf players actually hit the resultingsolid golf balls with a wood No. 1 club to examine their feeling of ballhitting. Results thereof are also shown in Table 2.

[0096] Methods for measuring the above-mentioned initial velocity,flight and hammering durability and a method for evaluating the feelingof ball hitting are as follows.

[0097] (1) Initial Velocity: A ball was hit at a head speed of 45m/second with a wood No. 1 club attached to a swing robot manufacturedby True Temper Co., and the initial velocity (m/second) of the ball atthat time was measured.

[0098] (2) Flight (carry): When a ball was hit at a head speed of 45m/second with a wood No. 1 club attached to a swing robot manufacturedby True Temper Co., Ltd., the distance (yard) to a position on which theball fell was measured.

[0099] (3) Hammering Durability: A ball was repeatedly collided againsta collision plate at a speed of 45 m/second, and the number ofcollisions required until the ball was broken was examined. Thehammering durability was indicated by the index taking as 100 the numberof collisions required until the ball of Comparative Example 4 wasbroken.

[0100] (4) Feeling of Ball Hitting: The feeling of ball hitting wasevaluated by an actual hitting test by ten top professional golfplayers. In the evaluation of the feeling of ball hitting, the feelingof ball hitting was compared with that of the ball of ComparativeExample 4, a conventional standard two-pieces solid golf ball. Criteriafor the evaluation are as follows and are indicated by the same symbolswhen they are shown in Table 2 to 4 as results of the evaluation. Inthis case, it indicates that 8 players of the ten players made the sameevaluation.

[0101] Criteria for Evaluation:

[0102] ∘: The feeling of ball hitting is softer and better than that ofthe ball of Comparative Example 4.

[0103] Δ: The feeling of ball hitting is equivalent to that of the ballof Comparative Example 4.

[0104] ×: The feeling of ball hitting is harder and worse than that ofthe ball of Comparative Example 4.

[0105] From the results shown in Table 2, the balls of Examples 1 to 6showed long flight (carry) and were excellent in durability, comparedwith the balls of Comparative Examples 1 to 3, and moreover, good in thefeeling of ball hitting, compared with the ball of Comparative Example4, a conventional standard two-pieces solid golf ball. TABLE 2Compounding Ratio (parts by Example Comparative Example weight 1 2 3 4 56 1 2 3 4 Polybutadiene HPB (A) 100 60 — — — — 30 — — — HPB (B) — — 10060 — — — — — — HPB (G) — — — — 100 60 — — — — HPB (H) — — — — — — — 10060 — BR11 — 40 — 40 — 40 70 — 40 100 Zinc Diacrylate 25 25 25 25 25 2525 25 25 25 Zinc Oxide 22 22 22 22 22 22 22 22 22 22 Dicumyl Peroxide1.8 1.8 1.8 1.8 1.8 1.8 1.8 1.8 1.8 1.8 Antioxidant* 0.5 0.5 0.5 0.5 0.50.5 0.5 0.5 0.5 0.5 Roll Processability 5 5 5 5 5 5 4 3 4 4 Weight (g)45.5 45.4 45.5 45.5 45.4 45.4 45.4 45.5 45.5 45.4 Compression (PGA) 9190 91 90 90 89 90 90 91 90 Initial Velocity (m/sec) 69.5 68.9 69.3 68.467.3 66.9 64.8 66.5 66.3 63.5 Flight (yard) 241 239 239 238 236 234 228234 231 222 Hammering Durability** 156 151 158 153 147 144 119 139 138100 Feeling of Ball Hitting ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ —

EXAMPLES 7 TO 12 AND COMPARATIVE EXAMPLES 5 TO 8

[0106] Compounding materials of each formulation shown in Table 3 werekneaded with a kneader and rolls to prepare a rubber composition. Theprepared rubber composition was charged into a mold, and crosslinked andmolded under pressure at 168° C. for 25 minutes to prepare a one-piecesolid golf ball having an outer diameter of 42.7 mm, which was formed ofa crosslinked molded article integrally molded. In Comparative Examples6 and 7, the modifier (polymeric type diphenylmethane diisocyanate)described in Japanese Patent Laid-open Publication (Hei) 11-164912 wasused. The ball obtained in Comparative Example 8 is a conventionalstandard two-pieces solid golf ball.

[0107] Results shown Table 3 indicate that the roll processability ofExamples 7 to 12 is excellent, compared with that of ComparativeExamples 5 to 8.

[0108] As to the one-piece solid golf balls thus obtained, their weight,compression (PGA indication), initial velocity, flight (carry) andhammering durability were measured, and the feeling of ball hitting wasevaluated, in the same manner as with Example 1. Results thereof areshown in Table 3. In the evaluation of the feeling of ball hitting, thefeeling of ball hitting was compared with that of the ball ofComparative Example 8, a conventional standard one-piece solid golfball.

[0109] As apparent from the results shown in Table 3, the balls ofExamples 7 to 12 showed long flight and were excellent in durability,compared with the balls of Comparative Examples 5 to 7, and moreover,good in the feeling of ball hitting, compared with the ball ofComparative Example 8, a conventional standard one-piece solid golfball. TABLE 3 Compounding Ratio (parts by Example Comparative Exampleweight) 7 8 9 10 11 12 5 6 7 8 Polybutadiene HPB (A) 100 60 — — — — 30 —— — HPB (B) — — 100 60 — — — — — — HPB (G) — — — — 100 60 — — — — HPB(H) — — — — — — — 100 60 — BR11 — 40 — 40 — 40 70 — 40 100 ZincDiacrylate 23 23 23 23 23 23 23 23 23 23 Zinc Oxide 25 25 25 25 25 25 2525 25 25 Dicumyl Peroxide 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6Antioxidant* 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Roll Processability5 5 5 5 5 5 4 4 4 4 Weight (g) 45.4 45.4 45.5 45.4 45.4 45.4 45.4 45.445.5 45.4 Compression (PGA) 81 80 81 81 80 80 80 81 80 80 InitialVelocity (m/sec) 64.1 63.9 64 63.9 63.3 62.8 60.7 62.3 62.1 60 Flight(yard) 226 225 225 224 220 219 213 218 217 211 Hammering Durability**157 156 156 155 148 145 119 144 141 100 Feeling of Ball Hitting ∘ ∘ ∘ ∘∘ ∘ ∘ ∘ ∘ —

EXAMPLES 13 TO 16 AND COMPARATIVE EXAMPLES 9 TO 11

[0110] Compounding materials of each formulation shown in Table 4 werekneaded with a kneader and rolls to prepare a rubber composition. Theprepared rubber composition was crosslinked and molded under pressure at150° C. for 30 minutes to obtain a core having a diameter of 38.5 mm.Then, the resulting core was coated by the injection molding method witha cover composition composed of a mixture of 100 parts by weight of anionomer resin (trade name: Surlyn, manufactured by du Pont) and 2 partsby weight of titanium dioxide to form a cover, thereby preparing atwo-pieces solid golf ball having an outer diameter of 42.7 mm.

[0111] As to the two-pieces solid golf balls thus obtained, theirweight, compression (PGA indication), initial velocity, flight (carry)and hammering durability were measured, and the feeling of ball hittingwas evaluated, in the same manner as with Example 1. Results thereof areshown in Table 4. In the evaluation of the feeling of ball hitting, thefeeling of ball hitting was compared with that of the ball ofComparative Example 4, a conventional standard one-piece solid golfball.

[0112] As apparent from the results shown in Table 4, also in thesetwo-pieces solid golf balls, the solid golf balls of Examples 13 to 16showed long flight and were excellent in durability, compared with thesolid golf balls of Comparative Examples 4 and 9 to 11, and moreover,good in the feeling of ball hitting, compared with the solid golf ballof Comparative Example 4, a conventional standard two-pieces solid golfball. TABLE 4 Example Comparative Example Compounding Ratio (parts byweight) 3 13 14 15 16 4 9 10 11 Polybutadiene HPB (B) 100 — — — — — — —— HPB (C) — 100 — — — — — — — HPB (D) — — 100 — — — — — — HPB (E) — — —100 — — — — — HPB (F) — — — — 100 — — — — BR11 — — — — — 100 — — — BR01— — — — — — 100 — — BR03 — — — — — — — 100 — BR18 — — — — — — — — 100Zinc Diacrylate 25 25 25 25 25 25 25 25 25 Zinc Oxide 22 22 22 22 22 2222 22 22 Dicumyl Peroxide 1.8 1.8 1.8 1.8 1.8 1.8 1.8 1.8 1.8Antioxidant* 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Roll Processability 5 54 5 4 4 3 3 3 Weight (g) 45.5 45.4 45.5 45.5 45.5 45.4 45.4 45.4 45.4Compression (PGA) 91 90 91 90 90 90 91 90 90 Initial Velocity (m/sec)69.3 67.4 71.8 66.9 66.7 63.5 61.9 61.3 68.7 Flight (yard) 239 237 247235 236 222 211 206 235 Hammering Durability** 158 144 165 142 141 10099 93 131 Feeling of Ball Hitting ∘ ∘ ∘ ∘ ∘ — Δ Δ Δ

What is claimed is:
 1. A rubber composition containing (a) 50 parts to100 parts by weight of polybutadiene rubber having a 1,4-cis bondcontent of 80 mol % to 100 mol % and a 1,2-vinyl bond content of 0 mol %to 2 mol %, which is modified with an alkoxysilyl group-containingcompound, (b) 0 part to 50 parts by weight of diene rubber other thanthe above-mentioned component (a) (wherein the total amount of component(a) and component (b) is 100 parts by weight), (c) 10 parts to 50 partsby weight of a crosslinkable monomer, (d) 5 parts to 80 parts by weightof an inorganic filler and (e) 0.1 part to 10 parts by weight of anorganic peroxide.
 2. The rubber composition according to claim 1,wherein modified polybutadiene rubber (a) has a ratio (Mw/Mn) of weightaverage molecular weight (Mw) to number average molecular weight (Mn) of1.0 to 3.5.
 3. The rubber composition according to claim 1, whereinmodified polybutadiene rubber (a) has a Mooney viscosity (ML₁₊₄ (100°C.)) of 30 to
 100. 4. The rubber composition according to claim 2,wherein modified polybutadiene rubber (a) has a Mooney viscosity (ML₁₊₄(100° C.)) of 30 to
 100. 5. The rubber composition according to claim 1,wherein component (a) is modified polybutadiene rubber obtained bypolymerizing butadiene using a rare earth element catalyst, andsubsequently allowing the alkoxysilyl group-containing compound to reactwith the resulting polybutadiene.
 6. The rubber composition according toclaim 2, wherein component (a) is modified polybutadiene rubber obtainedby polymerizing butadiene using a rare earth element catalyst, andsubsequently allowing the alkoxysilyl group-containing compound to reactwith the resulting polybutadiene.
 7. The rubber composition according toclaim 3, wherein component (a) is modified polybutadiene rubber obtainedby polymerizing butadiene using a rare earth element catalyst, andsubsequently allowing the alkoxysilyl group-containing compound to reactwith the resulting polybutadiene.
 8. The rubber composition according toclaim 4, wherein component (a) is modified polybutadiene rubber obtainedby polymerizing butadiene using a rare earth element catalyst, andsubsequently allowing the alkoxysilyl group-containing compound to reactwith the resulting polybutadiene.
 9. The rubber composition according toclaim 5, wherein the rare earth element catalyst is a neodymiumcatalyst.
 10. The rubber composition according to claim 6, wherein therare earth element catalyst is a neodymium catalyst.
 11. The rubbercomposition according to claim 7, wherein the rare earth elementcatalyst is a neodymium catalyst.
 12. The rubber composition accordingto claim 8, wherein the rare earth element catalyst is a neodymiumcatalyst.
 13. The rubber composition according to claim 1, wherein thealkoxysilyl group-containing compound is an alkoxysilane compound havingat least one epoxy group or isocyanate group in a molecule thereof. 14.The rubber composition according to claim 2, wherein the alkoxysilylgroup-containing compound is an alkoxysilane compound having at leastone epoxy group or isocyanate group in a molecule thereof.
 15. Therubber composition according to claim 3, wherein the alkoxysilylgroup-containing compound is an alkoxysilane compound having at leastone epoxy group or isocyanate group in a molecule thereof.
 16. Therubber composition according to claim 5, wherein the alkoxysilylgroup-containing compound is an alkoxysilane compound having at leastone epoxy group or isocyanate group in a molecule thereof.
 17. Therubber composition according to claim 9, wherein the alkoxysilylgroup-containing compound is an alkoxysilane compound having at leastone epoxy group or isocyanate group in a molecule thereof.
 18. Therubber composition according to claims 13, wherein the alkoxysilylgroup-containing compound is 3-glycidyloxypropyltrimethoxysilane. 19.The rubber composition according to claims 14, wherein the alkoxysilylgroup-containing compound is 3-glycidyloxypropyltrimethoxysilane.
 20. Asolid golf ball in which a part or the whole of a rubber substanceconstituting the solid golf ball is obtained by crosslinking and moldingthe rubber composition according to any one of claims 1 to 19.